One field of application for the invention is fixing bones in a desired position, for example to aid in healing of breaks or in the treatment of scoliosis or otherwise to correct abnormal curvatures of the spine. Other bone deficiencies and abnormalities may also benefit from the invention.
The spine is formed of superposed vertebrae, normally aligned along a vertebral axis, from the lumbar vertebrae to the cervical vertebrae, each having an anterior part, which is the vertebral body, and a posterior part, which is the vertebral arch (or neural arch), the anterior and posterior part, enclosing the vertebral foramen. Each vertebral arch is formed by a pair of pedicles and a pair of laminae, and transverse processes and/or a spinous process (or neural spine) project therefrom. The transverse processes and the spinous process project opposite to the vertebral foramen.
When the vertebrae are articulated with each other, the vertebral bodies form a strong pillar for the support of the head and trunk, and the vertebral foramen constitute a canal for the protection of the spinal cord (or medulla spinalis). In between every pair of vertebrae, there are two apertures, the intervertebral foramina, one on either side, for the transmission of the spinal nerves and vessels.
If the spine of a person has abnormal curvature, the vertebrae are typically inclined relative to one another and relative to said vertebral axis. The lateral edges of the vertebrae on one side are therefore closer together and define a concave outline, while the lateral edges on the other side are farther apart and define a convex outline.
In order to straighten the vertebral column as a remedy for this situation, the lateral edges of the vertebrae on the concave side can be moved away from one another and supported at distances from one another substantially equivalent to the distances between the lateral edges on the convex side. To do so, different kinds of devices may be used.
A first kind of device known in the art is a hook and rod system with hooks that are hooked on the internal wall of the vertebral foramen, and rods for connecting two or more hooks together, thereby holding the vertebrae in correct position relative to one another. Known examples of hook and rod system are disclosed, for instance, in the PCT Patent Application No. WO 2005/023126 and in U.S. Pat. No. 4,269,178.
However, using hooks can be difficult because their use increases the risk that the physician (or other operative) might contact and potentially damage the spinal cord that extends along the vertebral foramen (which can result in paralysis of the patient).
Another kind of known device is a screw and rod system with screws that are screwed into the vertebrae, and rods for connecting two or more screws together, thereby holding the vertebrae in correct position relative to one another. A known example of screw and rod system is disclosed, for instance, in the European Patent No. EP 157543381.
The screws typically have tulip-shaped heads and are inserted in pairs into the pedicles on each side of the spinous process on the posterior wall of the vertebrae. The screws therefore constitute fixing points on the vertebrae for holding the vertebrae. However, the screws are inserted into the pedicles of the vertebrae, which in some cases are small or have deteriorated and can be damaged or do not provide sufficient purchase to permanently hold the screw.
Another kind of known device is the system for fixing a bone to a rod, disclosed in WO 2009/047352. It comprises a rod, a blocking body and a conformable elongate member adapted to surround the bone, i.e. to form a loop around it. When using such a system, the elongate member is passed around said bone and through the blocking body and the rod is loaded into the blocking body. The ends of the elongate member are pulled so as to apply tension to the elongate member, and the elongate member and the rod are simultaneously fastened to the blocking body by means of the same fastening system, portion(s) of the elongate member being clamped between the rod and the blocking body. Such a system does not give complete satisfaction because it may be difficult to handle for the physician (or other operative). Indeed, the elongate member and the rod have to be fastened to the blocking body in one single step and, therefore, the elongate member needs to be kept under the desired tension while fastening the rod to the blocking body.